During the study period, there were 31,416 deaths, about 39 percent of which were due to breast cancer.

Women who were 50 years and older and who had tumors that were sensitive to the hormones estrogen and progesterone showed the biggest benefit from lumpectomy. They were 13 percent less likely to die from breast cancer, and 19 percent less likely to die from any cause, compared with those undergoing mastectomy.

For women who were under 50 with hormone-sensitive tumors, survival was about the same whether they received a lumpectomy or mastectomy, the researchers said.

Survival Benefit?

"It's good news in that a lot of women sometimes come in and feel that a mastectomy must be better than breast conservation," said Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, who was not involved in the study. But breast surgeons have had confidence based on previous studies that breast conservation surgery is equivalent to mastectomy for early stage cancer, Bernik said.

Bernik cautioned against interpreting the findings to mean that lumpectomy offers a survival benefit.

As a group, those who had mastectomies had some key differences compared with those who had a lumpectomy: They tended to have larger, more aggressive tumors, and they were more likely to have undergone surgery earlier in the study period, when treatment options were different, Bernik said.

Although the researchers tried to account for these differences by using statistics, "that's not perfect," Bernik said.

The researchers also could not directly determine whether participants had other conditions, besides breast cancer, that could have influence their survival.

Women who have lumpectomies need to be monitored in case their cancer reoccurs, an issue that may factor in to a women's decision to undergo the surgery, Hwang said. Because the study only looked at patient survival, it could not determine how likely lumpectomy patients were to have their cancer reoccur.